If you can jump off the ground on one leg then you have successfully exerted a force larger than your weight.
We can see this by just using Newton's second law: $\sum_i F_i=m a$,
where $F_i$ is one of the forces, $m$ is the mass of the person, and $a$ is the acceleration of the person
The two forces we are concerned with are the weight, $w$, of the person, which acts downward, and the force $L$ applied by the leg, which acts upward on the person (technically you exert a force $L$ downward on the ground, but by Newton's third law the ground also pushes up on you with force $L$).
I will leave it to you to apply Newton's second law to this system to see how we can get a positive (upward) acceleration.